Microstrip antennas are well known in the art. Typically, such an antenna employs an antenna patch which is a shaped conductive cladding on one side of a substrate having a similar cladding on the other side for a groundplane. A good starting point for a description of microstrip antennas can be found in a publication of the IEEE Antennas and Propagation Society of Jan. 1981, Volume AP-21.
In a microstrip antenna, the radiating element is often referred to as the patch. The shape of the patch and its feedpoint determine such characteristics as polarization, SWR (standing wave ratio) and the radiation pattern.
The substrate material, plays an important role in the performance of a microstrip antenna because of its dielectric constant. When temperature changes affect the dielectric constant, the center resonant frequency of the patch tends to shift. If the passband for a receiving patch is, for example, about 2% for a 1.575 gigaherz center frequency, then a change of the dielectric constant as a result of temperature changes or production manufacturing techniques can shift the passband too much. This could result in an excessive undesirable deterioration in the performance of the patch antenna.
Various materials and techniques are used to improve the dimensional stability of the microstrip antenna. PTFE, polytetrafluorethylene, tends to be the material of choice, particularly when it is combined with a glass fiber in a woven web to improve stability or with a glass randomly oriented fiber.
PTFE has excellent characteristics at the frequencies of interest, but tends to be expensive. Use of random fibers tends to result in unpredictable voids and an uneven dielectric constant in the substrate and thus an uneven performance from one patch antenna to the next.
The design of a patch antenna must take the feed line into consideration because of the way it influences excitation of the edges and the impedance represented to the input or output circuit. When the patch antenna is employed as a receiver, it is particularly important that a proper impedance match is obtained at the feed point which should be at a location which is compatible with the layout of underlying receiver circuitry.
Various feed lines have been described, including feed lines which extend to particularly desired excitation regions inside a patch by way of undercuts made in the patch alongside a feed line. U.S. Pat. No.. 4,692,769 shows one form of such undercut with a feedline for a rectangular patch. FIG. 5 in U.S. Pat. No.. 4,067,016 shows a pair of undercut feed lines applied to the corners of a square-shaped patch.
Other feeds for patch antennas are shown in U.S. Pat. Nos. 4,197,544 and 4,051,478.
When a patch antenna is employed for receiving signals from so-called GPS global position system satellites, the signal strength at the ground tends to vary with very weak levels occurring from time to time. Proper reception requires a low noise figure to provide reliable operation in a wide range of field conditions. Low noise figures are difficult to achieve, particularly when a broad range of ground conditions must be considered, such as wide temperature swings, exposure to sunlight, rain, etc.